Add _toggle to controlpotmeter controls and keyboard mappings

(this is a feature, not a bug)

The following patch (against trunk, rev 2988) adds a few useful _... extensions to ControlPotMeter controls (volume, crossfader, ...).
Adds the following functions and control bindings:
* X_zero: sets the value to 0.0
* X_one: sets the value to 1.0
* X_minus_one: sets the value to -1.0
* X_toggle: sets the value to 0.0 if the value was > 0.0, and to 1.0 if the value was 0.0
* X_minus_toggle: sets the value to -1.0 if the value was > -1.0, and to 1.0 if the value was -1.0

What you can effectively use this for:
* in your custom keybindings (~/.mixxx/Custom.kbg.cfg) bind a key to e.g. "volume_toggle" in [Channel1] and to another key in [Channel2], like this:

...
[Channel1]
...
volume_toggle F9
...
[Channel2]
...
volume_toggle F10
...

* use those keys to cut off or one once of the tracks while you're playing two tracks (mixed)

It's a classic trick used by DJs ;)

Another trick I'm using this for is to switch the crossfader from left track to right track (also while playing both):

...
[Master]
...
crossfader_minus_toggle F12
crossfader_zero Shift+F12
...

With F12 I can tilt the crossfader from left (-1.0) to right (1.0), and press Shift+F12 to put it in the middle (0.0) again.

Ideally, the "toggle" ones would remember the previous value and switch back to that, but that would introduce a few changes I wasn't comfortable doing as I don't know the codebase very well. And as long as you're using the gain to change the volume rather than the per-channel volume sliders, it works perfectly well this way.

Latest state of the patch can also be tracked here:
https://pmbs.links2linux.org/package/view_file?file=mixxx-add_toggleValue.patch&package=mixxx-unstable&project=Multimedia